This invention relates generally to work transfer devices and is more specifically concerned with a work transfer device of the type which repeatedly executes an operating cycle of motion in a closed, generally rectilinear path to progressively advance workpieces from station to station.
In automation machining it is a conventional practice to position a plurality of machine tools in a series path and to progressively move workpieces along this path for machining operations. One type of work transfer device for progressively advancing workpieces repeatedly executes an operating cycle consisting of motion in a closed, generally rectilinear path. This type of device generates motion along two perpendicularly related axes, one longitudinal, the other transverse. A typical cyclical sequence for progressively advancing workpieces involves a longitudinal transfer stroke for advancing each workpiece longitudinally from one machining station to the next, a transverse retraction stroke for releasing the workpieces at their new stations, a longitudinal return stroke to prepare for the next transfer, and a transverse extension stroke to once again engage the workpieces for the next longitudinal transfer stroke. This rectilinear cycle is repeatedly executed whereby workpieces are progressively advanced through successive machining operations.
Typically, the longitudinal motion axis is horizontal while the transverse motion axis is either horizontal or vertical. In the case of a vertical transverse motion axis the work transfer device may be of a type commonly referred to as a walking beam transfer device. The walking beam comprises a horizontal beam which spans the length of the series path and on which workpiece engaging members are mounted at uniformly spaced longitudinal distances equal to the length of the longitudinal stroke. The stations are similarly uniformly longitudinally spaced. The walking beam repeatedly executes the above described closed loop operating cycle to progressively advance the workpieces from station to station.
In the prior work transfer devices of this type, motion for each axis is derived from a rotary cam, such as a barrel cam. The cams are driven from a prime mover such as an electric or hydraulic drive. Followers are engaged with the cams and operably coupled with the walking beam via appropriate mechanisms to impart the desired rectilinear motion to the workpiece engaging members. Typically, the cams are rotated in unison from a common prime mover and the phasing between cams is such as to impart the desired rectilinear motion to the workpiece engaging members.
In the case of a lift and carry type transfer, which would be executed by a walking beam type device, one cam controls the horizontal longitudinal motion while the other cam controls the vertical transverse motion. Where both cams are barrel cams mounted on a common shaft the shaft may be arranged parallel to one of the two motion axes, say for example the horizontal longitudinal axis. The throw, or profile, of the horizontal cam is therefore also longitudinal, and the mechanism for coupling the horizontal cam with the walking beam to impart horizontal longitudinal motion to the walking beam is not especially complex. Such a mechanism may simply be a longitudinally shiftable carriage having a cam follower following the horizontal cam profile. Such a mechanism will, for a given cam profile, possess a fixed longitudinal stroke corresponding exactly to the throw of the cam. Alternatively, other mechanisms may be used to provide adjustable stroke length.
However, the throw of the vertical cam is also horizontal. Consequently a more complex mechanism is required in order to couple the vertical cam with the walking beam to effect vertical stroking. One prior mechanism comprises a bellcrank to transform the horizontal throw or profile of the vertical cam into vertical motion of the walking beam. For this purpose the bell crank is mounted on the work transfer device for rocking motion about a pivot axis. One arm of the bellcrank is coupled to the vertical cam while the other arm is arranged at an angle to the first arm. The bellcrank converts the generally horizontal rocking motion of the first arm into generally vertical rocking motion of the second arm. Because the walking beam must move vertically at both the beginning and at the end of the longitudinal stroke it is necessary for the coupling between the second crank arm and the walking beam to accommodate the full longitudinal stroke imparted to the walking beam by the horizontal cam.
One previous construction for accomplishing this involves a horizontal track which is bodily shifted longitudinally with longitudinal stroking of the walking beam. The track is also bodily shiftable vertically, and the walking beam is vertically supported on the track. The second crank arm of the bellcrank comprises a roller which rides in the track. The track is long enough that the roller remains in the track throughout the full longitudinal stroke. When the walking beam is at either end of its longitudinal stroke, the vertical cam is effective to rock the bellcrank which in turn via the engagement of the roller with the track is effective to impart the corresponding vertical transverse motion to the track, and hence also to the walking beam.
This prior construction possesses a number of disadvantages. One disadvantage is that the track shifts longitudinally with the longitudinal stroke and hence, the overall longitudinal dimension for the device must accommodate both the longitudinal stroke plus the length of the track. This can impose constraints on the layout of the automated line resulting in undesirable inefficiencies.
Another disadvantage is that the weight carrying capacity is limited due to the requirement that the bellcrank be capable of supporting the weight at both ends of the longitudinal stroke. Because the moment acting about the pivot axis of the bellcrank due to the combined weights of the walking beam and workpieces is greater when the walking beam is at one end of its longitudinal stroke than at the other end, the weight carrying capacity is established when the walking beam is at that one end. Obviously, this is less than the weight carrying capacity that would be obtained if the moment with the beam at one end were the same as the moment with the beam at the other end.
Prior walking beam mechanisms present a problem where a relatively long transfer stroke is involved, say 16 inches or more, or where the overall length of the transfer line is relatively long. There may arise a need for a plurality of cam operated transfer devices, and these can involve higher cost, larger space requirements, and more complicated adjustment. It becomes difficult to synchronize the devices so as to produce the desired simultaneous transfer of workpieces from station to station.
The present invention provides a new and improved work transfer device which overcomes the foregoing disadvantages of prior work transfer devices. With the present invention the overall longitudinal dimension of such a work transfer device can more closely correspond to the length of the longitudinal transfer stroke. This yields greater efficiencies in the layout and operation of an automated line using the invention in association therewith. Furthermore, the invention avoids the weight carrying limitations inherent in prior work transfer devices of the type described above. The invention is particularly well suited for walking beam type transfer devices although its principles are not limited to that specific type. The invention provides a more efficient arrangement of component parts within the work transfer device, and it also provides the capability for slaving additional work transfer devices to a master device without the problem of having to synchronize cams of one device to those of other devices. The master device contains the two cams for imparting the longitudinal and the transverse motions. The slave units do not contain any such cams and hence may be substantially compact. Yet the slave units incorporate principles of the invention constituting improvements over prior work transfer devices.
The foregoing features, advantages, and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time for carrying out the invention.